The light colored minerals are dominantly plagioclase of
andesine composition and may include sodalite, and nepheline.
Euhedral plagioclase (< 3.0 mm) displays core compositions ranging from
andesine to labradorite ([An.sub.33-48]; Table 2, Fig.
Plagioclase with
andesine to labradorite and clinopyroxene with augitic composition are predominant phases in these volcanic rocks.
Andesine to oligoclase ([An.sub.27-30]) predominate over sodic sanidine and scarce clinopyroxene and titanomagnetite in the matrix of fresh trachyandesite facies.
These data indicate that the plagioclase has An values of 23.02-36.15 and that the granodiorite contains oligoclase (An = 23-28) and
andesine (An = 30-36) plagioclase (Fig.
Plagioclase is cloudy to saussuritized in some rocks especially in the more calcic cores but where fresh it is of intermediate anorthite content (calcic oligoclase to
andesine).
Most of them are classified as labradorite and
andesine (Fig.
Tabular plagioclase shows weak zoning with more sodic rim (oligoclase to
andesine) and more calcic cores (basic
andesine).
In vogesite its composition ranges from
andesine to bytownite, and minor albite occurs in the groundmass of monchiquite (Lindgren and Ransome, 1906; Loughlin and Koschmann, 1935; Thompson, 1992).
Other plagioclase grains have distinct cores of higher An content (of
andesine composition), which are commonly seriticized (Fig.
Euhedral to subhedral zoned oligoclase to
andesine plagioclase (30% to 40%) and anhedral quartz (20% to 30%), in small pods, are the primary constituents of the rock.
Zoned plagioclase with average composition of
andesine is the most abundant mineral, with less abundant interstitial K-feldspar and quartz.